Question: In math class, the girl to boy ratio is $4$ to $5$. If there are $20$ girls, how many boys are there?
A ratio of $4$ girls to $5$ boys means that there is a group of $4$ girls for every group of $5$ boys. If there are $20$ girls, then there are $5$ groups of $4$ girls each. Because we know the ratio is $4:5$ , there must be $5$ groups of $5$ boys each. There is a total of $25$ boys in math class.